In a serial telemetry data link, one method of encoding the data is to group data words into logical units preceded by a link synchronization data word, which logical units are called frames. A particular format may consist of a fixed number of frames which are collectively called a paragraph, each frame being a subframe of the paragraph. In order to make meaningful use of data at the receiving node in a network, it is necessary to know the transmitted data format or at least the location of the data words of particular interest. Next it is necessary to have equipment which can extract or decommutate the particular data words from all the others in the data stream.
In the past, systems used to decommutate serial digital data and control the data output to multiple peripheral devices made extensive use of comparator circuits to test a desired data word address versus the real time data word address. Generally, this means a comparator circuit and a latched reference register are required for each data word request. Alternatively, one comparator may be used with extensive multiplexing of one input. The selection of data as either most significant bit (MSB) or least significant bit (LSB) first is implemented by a switch or patch panel and defines the order of all data to be delivered to the peripheral devices. Similarly, the number of bits and location of the data word is usually either set for all devices or is set by switches or a patch panel for each device. Previous use of computers has been confined to setting up latched registers for comparator test (hardware or software) reference and setting MSB/LSB or other control parameters which determined the mode of operation of the entire decommutator.
The prior art decommutation systems have several disadvantages. Usually the selection of bit strings within a data word is not available. The selection of MSB or LSB interpretation of data words applied to all data words. Real time alteration of data word requests is generally available only with a significant increase in hardware.
It is therefore an object of the present invention to provide a decommutation system wherein the complete configuration of the decommutation information can be changed in real time to accommodate dynamic requirements.
It is another object of the invention to provide a decommutation system wherein any data word requested by any peripheral device can be either MSB or LSB first and any sequence of consecutive bits in a data word or overlapping more than one data word can be selected for any peripheral device.